Added a TaskSetManager unit test.

This test ensures that when there are no
alive executors that satisfy a particular locality level,
the TaskSetManager doesn't ever use that as the maximum
allowed locality level (this optimization ensures that a
job doesn't wait extra time in an attempt to satisfy
a scheduling locality level that is impossible).

@mateiz and @lirui-intel this unit test illustrates an issue
with #892 (it fails with that patch).

Author: Kay Ousterhout <kayousterhout@gmail.com>

Closes #1024 from kayousterhout/scheduler_unit_test and squashes the following commits:

de6a08f [Kay Ousterhout] Added a TaskSetManager unit test.